The dimming of artificial illumination is beneficial for energy conservation, since it enables many users to reduce light levels to suit individual preferences. Also, occupancy sensors in commercial establishments can run on dim when no customers are nearby. Further, dimming is integral for a lighting backup in day-lighting systems of large buildings. On partly cloudy days, fluctuating solar illumination is compensated for by responsive artificial lighting that varies in opposition to changes in sunlight to produce constant illuminance. Finally, smart electrical grids of the future may incorporate dimming of lights to handle variable loads or emergency situations.
As LEDs replace incandescent lighting, retrofitting will result in many LED lamps being connected to pre-existent dimmer switches. The most prominent types of dimmer switch are the rheostat, which simply lowers the line voltage, and the triac, which acts as a phase dimmer by zeroing out the line voltage during a given fraction of each AC cycle. In either case, a direct AC-to-DC conversion from incoming dimmed AC power to the DC power required by LEDs will have disadvantages. As rheostat output voltage is reduced, the current through the LEDs is reduced in proportion, and at low enough levels the current through the LEDs will fall below their threshold level, which can damage them. With the triac, the LEDs are off for part of each AC cycle, resulting in a perceptible flicker and possibly some audible noise.
When LEDs have their own power supply in a new installation, dimming is usually accomplished by pulse-width modulation, which is conceptually similar to a triac's phase dimming of AC line voltage, except that the modulation is then typically at too high a frequency to be seen by the human eye. The line power to such a new installation would never have a standard dimmer on it. In retrofits however, it would be advantageous to avoid the costs of replacing a pre-existent dimmer, since the decision to retrofit in the first place is highly cost-sensitive. This places a premium on adaptable power supplies for LED lighting.
Although LEDs are specifically described, the principles of the present application can of course be applied to other forms of light sources, including light sources hereafter to be developed.